Microstructure and physical features of the HAZ

Near the fusion zone, the phase structure of base metal is coarse as a result of the high temperature of the base metal during welding. In multi-run welding, ICCGHAZ (intercritically reheated coarse grained heat affected zone) is the worst area in the base metal (Li et al. 2001; Kim et al. 1991; Davis & King 1993).

Both heat input and t8/5 (cooling time from 800 °C to 500 °C) time change the microstructure of the welded base metal and these two factors must be under control while welding. There are numerous recommendations from manufacturers regarding heat input and t8/5 time. The main differences between recommendations relate to preheating and post-heating. In specifications, however, there are also differences in spotheating temperature. Using recommended values, it is possible to successfully weld HSS.

In the study done by Kaputska et al. (2008), it was concluded that the fusion zone microstructure and hardness were found to be affected by the base metal chemistry, the cooling rate conditions, and the filler metal composition.

The elongation of the welded structure decreases as the yield strength of HSS grows. Yasuyama et al (2007) compares steels with yield strengths ranging from 270 MPa to 980 MPa. In the study, steels were welded by the YAG laser, mash seam, and plasma arc methods. It was confirmed that the elongation of the weldment declined compared to that of the base metal, regardless of the base metal strength. This was determined by conducting a tensile test both parallel and perpendicular to the weld line. It was therefore concluded, that the elongation is very low in high strength welded structures (Yasuyama et al.

2007).

32

Lambert et al. (2000) studied the microstructure of the martensite-austenite constituent in HAZ of HSLA steel welds in relation to toughness properties. The material used in the research was HSLA steel, with a yield strength of 433 MPa.

Charpy impact test results indicated that the correlation between the toughness and microstructure of low carbon steel simulated HAZs is rather complex. The amout of M-A constituents and coarseness of bainite are major metallurgical factors affecting the impact properties (Lampert et al. 2000). In the same study, Lampert et al. (2000) also noticed that retained austenite and low carbon transformed martensite have significantly different influences on cleavage fracture and impact properties of simulated HAZ microstructure, where freshly transformed high carbon martensite is much more deleterious than retained austenite.

Metallographic investigations demonstrated the existence of different M-A constituents. In the most brittle zones (the ICCGHAZ), retained austenite was mostly located between bainitic packets, whereas blocky martensite and mixed M-A constituents were located at prior austenite grain boundaries. In mixed M-A constituents, austenite was distributed at the periphery, while martensite was located at the centre. This distribution of retained austenite could be a result of chemicals and/or the mechanical stabilization mechanism (Lambert et al. 2000).

Furthermore, through TEM, Lambert et al. (2000) found a constituent retained austenite at room temperature. The presence of constituent may influence the thermal stability of retained austenite, as they propagate before transformation.

These observations constitute preliminary investigations of the transformation mechanism of retained austenite islands.

Moon et al. (2000) compared two new ultra-low-carbon matching filler metals, with HY steel (High yield, quenched and tempered, steel) of HSLA steel.

Despite the low heat input, 1.2 kJ/mm, the fusion zone hardness of two of the new ultra-low-carbon matching filler metals are comparable to the base metal hardness. The results were achieved through researching the microhardness variations in the weld and HAZ areas and corresponding this with the

33

microstructure of the weldment. In addition, the heat affected zone of the base metal was the hardest region in each weldment examined, regardless of filler metal type, base metal, or heat input. The maximum hardness occurs about midway through the HAZ of each weldment studied, rather than adjacent to the fusion boundary (Moon et al. 2000).

Additionally, Moon et al. (2000) studied that the fusion zone consists predominantly of lath ferrite with varying amounts (depending on location) of untempered fine lath martensite, as well as small amounts of interlath retained austenite and oxide inclusions. No polygonal ferrite or solid-state precipitates such as carbides or carbonitrides were observed in the fusion zone. The local variations in microhardness correlate well with the local variations in the microstructure.

Research carried out to study the research done by Mohandas et al. (1999) has displayed that the high Ms and Bs temperatures of steel are also responsible for low softening tendency. Steel, which has longer critical cooling time for full martensite transformation, exhibited greater resistance for softening with high heat inputs.

In the investigation of heat input it was realized that the number and morphology of the ML (lath martensite) in the HAZ had some variations under different weld heat inputs (E= 0.92 ~ 1.86 kJ/mm). The carbon gathers near the grain boundary and then becomes a carbide with Fe, Mn, Mo etc. so that the impact toughness decreases. The carbide has strong direction bonds with the lath microstructure which provides the low energy passage for the impact fracture and increases brittle crack sensitivity. The fine precipitate distributed inside the grain or at the boundary is favorable to improve toughness. By controlling weld heat input (E ≤ 2.0 kJ/mm), the presence of carbides in the HAZ can be removed, and therefore the impact toughness in this zone can be assured. It was also indicated from the test results of Juan at al. (2003) that the cooling time (t8/5) should be controlled (t8/5 10-20 s) to improve toughness in the HAZ. This is so, because the cooling time increases with larger weld heat

34

inputs, which increases the potential for the deterioration of impact toughness in the HAZ (Juan et al. 2003).

When welding ultra HSS, with a yield strength of more than 900 MPa, with MAG welding, it is important to precisely and accurately control heat input to the lowest possible temperatures. Zeman (2009b) examined ultra HSS, with a yield strength of 1100 MPa. In the case of the joint made by the MAG method, the weld is characterized by its bainitic structure. In the HIZ (Heat Impact Zone), Zeman observed a purely martensite structure or mixture of bainite and martensite structures (Zeman 2009b). In the same study, Zeman (2009b) noticed that ultra HSS requires the linear energy of welding to be precisely adjusted. If the linear energy of welding is too low, there could be excessive hardening of the HIZ, which increases the risk of cold cracking, whereas if the linear energy of welding is too high, the strength properties can decrease.